At least one-quarter of the world's known mammal species are at risk of becoming extinct, and about half are declining in population, a global survey released Monday morning has found.

The study – which took 1,700 experts in 130 countries more than five years to complete – was conducted as part of the International Union for Conservation of Nature's Red List of Threatened Species and will be published this week in the journal Science. It covers all of the 5,487 wild-mammal species known to humans since the year 1500, and is the most thorough study of its kind since 1996.

For some species, data were quite detailed, down to individual breeding rates. But many others remain virtually unknown.... Particularly sparse were data on some rodents and bats, several of which are known just from single museum specimens collected from remote regions. In all, 836 species were so poorly studied that it was impossible to tell what their conservation needs might be.

That means that the number of species actually threatened could be as high as 36 percent.

Land and marine mammals face different threats, the scientists said, and large mammals are more vulnerable than small ones. For land species, habitat loss and hunting represent the greatest danger, while marine mammals are more threatened by accidental killing through fishing bycatch, ship strikes, and pollution.

While large species such as primates (including the Sumatran orangutan and red colobus monkeys) and ungulates (hoofed animals) may seem more physically imposing, the researchers wrote that these animals are more imperiled than small creatures such as rodents or bats because they "tend to have lower population densities, slower life histories, and larger home ranges, and are more likely to be hunted."

Primates face some of the most intense pressures: according to the survey, 79 percent of primates in South and Southeast Asia – including the Hainan gibbon – are facing extinction.

Some species have made a recovery in recent years. The black-footed ferret, for instance, is no longer extinct in the wild after the US Fish and Wildlife Service mounted an extensive captive-breeding program and successfully reintroduced it into the wild in eight Western states. This ferret, which eats prairie dogs, now appears to have three small but self-sustaining populations in South Dakota and Wyoming.

But the successful recovery efforts are not nearly enough to make up for the declines in other species. "Fifty percent of species are declining and 5 percent of species are in an upward recovery — that's just not enough," Jan Schipper, the mammologist who coordinated the study for the IUCN, told Scientific American.

"The financial crisis is nothing compared with the environmental crisis," the deputy head of IUCN's species programme, Jean-Christophe Vie, told BBC News.

"It's going to affect a few people, whereas the biodiversity crisis is going to affect the entire world. So there is a risk that because of the financial crisis, people are going to say 'yeah, the environment is not that urgent'; it is really urgent."